1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device and more particularly, to a method of fabricating a semiconductor device having a capacitor equipped with a dielectric film made of a high dielectric-constant or ferroelectric material.
2. Description of the Related Art
A conventional semiconductor memory device Including a storage capacitor equipped with a ferroelectric film as a capacitor dielectric is disclosed In the Japanese Non-Examined Patent Publication No. 7-50391 published in February 1995. In this memory device, the storage capacitor is implemented by using conventional fabrication processes or techniques for silicon-based semiconductor integrated circuit devices.
This conventional memory device utilizes the residual polarization of a ferroelectric film for storing the information. The ferroelectric film is applied with a positive or negative bias voltage to thereby cause polarization in the ferroelectric film. The polarization thus caused in the ferroelectric film is left due to the residual polarization even after the application of the bias voltage is stopped, This means that this memory device serves as a non-volatile memory.
FIG. 1 shows the configuration of the conventional semiconductor memory device disclosed in the Japanese Non-Examined Patent Publication No. 7-50391.
In FIG. 1, an isolation insulating film 102 is formed on a single-crystal silicon substrate 101 to define an active region. In the active region, a source region 104a and a drain region 104b are formed in the substrate 101, and a gate electrode 105 is formed over the substrate 101 through a gate insulating film 103 between the source and drain regions 104a and 104b, thereby forming a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET).
An interlayer insulating film 106 is formed to cover the MOSPET and the isolation insulating film 102.
A lower electrode 107 of a storage capacitor is formed on the interlayer insulating film 106. A ferroelectric film 108 of the storage capacitor is formed on the lower electrode 107 to be partially overlapped therewith. An upper electrode 109 of the storage capacitor is formed on the ferroelectric film 108 to be entirely overlapped therewith.
A first protection film 115 is formed on the interlayer insulating film 106 to cover the storage capacitor and the MOSFET.
A metallic wiring film 113a is formed on the first protection film 115 to be electrically connected to the upper electrode 109 of the capacitor through a contact hole 111a and the source region 104a of the MOSFET through a contact hole 112a. The contact hole 111a penetrates the first protection film 115 alone. The contact hole 112a penetrates the first protection film 115 and the interlayer insulating film 106.
A metallic wiring film 113b is formed on the first protection film 115 to be electrically connected to the lower electrode 107 of the capacitor through a contact hole 111b. The contact hole 111b penetrates the first protection film 115 alone.
A metallic wiring film 114 is formed on the first protection film 115 to be electrically connected to the drain region 104b of the MOSFET through a contact hole 112b. The contact hole 112b penetrates the first protection film 115 and the interlayer insulating film 106.
Asilicon dioxide (SiO2) subfilm 116a, which is doped with phosphorus (P), is formed on the first protection film 115 to cover the metallic wiring films 113a, 113b, and 114. Another SiO2 subfilm 116b, which is not doped with phosphorus, is formed on the SiO2 subfilm 116a. These two SiO2 subfilms 116a and 116b constitute a second protection film 116.
As the first protection film 115, a silicon dioxide (SiO2) or silicon nitride (SiNx) is typically used. A silicon oxide or silicon nitride film is usually formed by a Chemical Vapor Deposition (CVD) process using a gaseous source, a sputtering process using a solid source, or a coating and sintering process using a liquid source.
With the CVD process for SiO2 or SiNx using a gaseous source, a source gas for silicon (Si) tends to contain hydrogen (H) or hydrogen compound chemically bonded with silicon serving as a film formation species. A typical example of the source gas for silicon is mono-silane (SiH4). The source gas for Si is usually decomposed during the CVD process with the use of heat or plasma.
It has been known that a lot of activated hydrogen (i.e., hydrogen radical) tends to be generated in an atmosphere during a gas-source CVD process using a hydrogen-containing source gas for silicon, and the activated hydrogen thus generated reduces the ferroelectric film 108 of the storage capacitor to thereby degrade the performance or characteristics of the capacitor.
The effect of hydrogen to lanthanum-doped lead zirconate titanate (PZT, PbZr1xe2x88x92xTixO3), i.e., PLZT, was reported in an article, International Electron Devices Meeting (IEDM), Technical Digest, December 1994, pp. 337-340, which was written by R. Khamankar et al. and entitled xe2x80x9cIMPACT OF POST PROCESSING DAMAGES ON THE PERFORMANCE OF HIGH DIELECTRIC CONSTANT PLZT THIN FILM CAPACITORS FOR ULSI DRAM APPLICATIONSxe2x80x9d.
This article describes the effect of hydrogen, nitrogen (N2) plasma, and x-ray to a semiconductor memory device equipped with a ferroelectric storage capacitor including a PLZT film, and the polarization degradation of the PLZT film and the leakage-current increase of the storage capacitor. This article also describes the repair of the damage or degradation of the PLZT film or capacitor thus caused by a specific thermal annealing process.
FIG. 2 shows the relationship of the polarization degradation Qcxe2x80x2 of the PLZT film of the hydrogen-damaged device with the bias voltage applied thereto while using the annealing temperature as a parameter, The memory device is exposed to a forming gas made of 5% hydrogen (H2) and 95% nitrogen (N2), and is damaged due to hydrogen in the forming gas. The thermal annealing process is performed in an atmosphere containing nitrogen (N2) or oxygen (O2). The word xe2x80x9cFRESHxe2x80x9d in FIG. 2 means the case where the memory device 19 not damaged due to exposure to hydrogen.
FIG. 3 shows the relationship of the leakage current density of the storage capacitor of the damaged device with the lanthanum (La) concentration of the PLZT film. The memory device is exposed to the H2/N2 forming gas, N2 plasma, or x-ray. The word xe2x80x9cFRESHxe2x80x9d in FIG. 3 means the case where the memory device is not damaged due to exposure to hydrogen, plasma, nor x-ray.
Since each of PZT and PLZT is a composite metallic oxide, it tends to be reduced by activated hydrogen contained in the atmosphere. Due to this reduction, oxygen is released from the matrix of the oxide to thereby form defects. As a result, electrons tend to become unstable due to the defects thus formed, degrading the electric insulating capability. This leads to the decrease in polarization and increase in leakage current.
To form the contact holes 111a and 111b penetrating the protection film 115 in the conventional semiconductor memory device shown in FIG. 1, the protection film 115 needs to be etched by a wet process using a liquid such as an acid or a dry process using plasma. It is needless to say that the dry process is preferred to the wet process, because of its higher fabrication yield.
In a conventional dry etching processes, a fluorocarbon-system gas is typically used as an etching gas. For example, to ensure a satisfactorily high selection ratio between silicon and silicon oxide, it is typical that the etching gas contains hydrogen. For example, trifluoromethane (CHF3) alone or a mixture of trifluoromethane and hydrogen is often used.
Similar to the above-described case of the gas-source CVD process using a hydrogen-containing source gas for silicon, activated hydrogen tends to be generated in an etching atmosphere, and the activated hydrogen reduces the ferroelectric film 108 of the storage capacitor. As a result, the polarization of the ferroelectric film 108 is decreased and at the same time, the leakage current of the storage capacitor is increased and the dielectric breakdown resistance thereof is lowered.
Accordingly, an object of the present invention is to provide a method of fabricating a semiconductor device with a capacitor that prevents the performance degradation of the capacitor that may be caused during the fabrication process sequence.
Another object of the present invention is to provide a method of fabricating a semiconductor device with a capacitor that prevents the leakage current from increasing and the dielectric breakdown resistance from decreasing during a CVD or dry etching process for forming an insulating film to cover the capacitor.
Still another object of the present invention is to provide a method of fabricating a semiconductor device with a ferroelectric capacitor that prevents the polarization of the ferroelectric capacitor from degrading.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
A method of fabricating a semiconductor device according to a first aspect of the present invention is comprised of the following steps (a) to (d):
(a) A lower electrode of a capacitor is formed on a first insulating film. The first insulating film is typically formed on or over a semiconductor substrate.
(b) A dielectric or ferroelectric film of the capacitor is formed on the lower electrode to be overlapped therewith.
(c) An upper electrode of the capacitor is formed on the dielectric or ferroelectric film to be overlapped therewith.
(d) A second insulating film is formed to cover the capacitor by a thermal CVD process in an atmosphere containing no plasma at a substrate temperature in which hydrogen is prevented from being activated due to heat.
A source material of the second insulating film has a property that no hydrogen is generated in the atmosphere through decomposition of the source material during the thermal CVD process.
With the method of fabricating a semiconductor device according to the first aspect of the present invention, the second insulating film is formed to cover the capacitor by a thermal CVD process in an atmosphere containing no plasma at a substrate temperature in which hydrogen is prevented from being activated due to heat. The source material of the second insulating film has a property that no hydrogen is generated in the atmosphere through decomposition of the source material during the thermal CVD process.
As a result, the dielectric or ferroelectric film is not reduced by the activated hydrogen existing in the atmosphere during the thermal CVD process. Thus, the leakage current is prevented from increasing and the dielectric breakdown resistance is prevented from degreasing during the CVD process for forming the second insulating film to cover the capacitor. This means that the performance degradation of the capacitor is prevented from occurring.
Moreover, when the capacitor has a ferroelectric film, in other words, the capacitor is a ferroelectric capacitor, the dielectric or residual polarization of the forroelectric capacitor is prevented from degrading. This is also because the dielectric or ferroelectric film is not reduced by the activated hydrogen existing in the atmosphere during the thermal CVD process.
In a preferred embodiment of the method according to the first aspect of the present invention, the second insulating film is SiO2, and the substrate temperature is in a range of 300 to 500xc2x0 C.
When the substrate temperature is lower than 300xc2x0 C., the SiO2 film tends to contain a large amount of water, degrading the quality of the SiO2 film. When the substrate temperature is higher than 500xc2x0 C., the deposition or growth rate of the SiO2 film is excessively low and the step coverage tends to degrade.
As the source material of the second insulating film of SiO2.
tetraethyl orthosilicate (TEOS) [Si(OC2H5)4],
hexamethyldisiloxane [(CH3)3SiOSi(CH3)3],
diacetoxydibutoxysilane [Si(OC3H7)2(OCOCH3)2], or
tetraisocyanatesilane Si(NCO)4 may be preferably used.
In another preferred embodiment of the method according to the first aspect of the present invention, the second insulating film is SiNx, and the substrate temperature is in a range of 500 to 750xc2x0 C.
When the substrate temperature is lower than 500xc2x0 C., the deposition or growth rate of the SiNx film is excessively low.
When the substrate temperature is higher than 750xc2x0 C., there arises a possibility that hydrogen is activated by heat to thereby cause reduction of the SiNx film.
As the source material of the second insulating film of SiNx, silicon diamide complex [(Si(NMe2)4xe2x88x92nHn], where n is zero or a positive integer (i.e., 0, 1, 2, . . .) may be preferably used.
A method of fabricating a semiconductor device according to a second aspect of the present invention is comprised oft he following steps (a) to (e):
(a) A lower electrode of a capacitor is formed on a first insulating film. The first insulating film is typically formed on or over a semiconductor substrate,
(b) A dielectric or ferroelectric film of the capacitor is formed on the lower electrode to be overlapped therewith.
(c) An upper electrode of the capacitor is formed on the dielectric or ferroelectric film to be overlapped therewith.
(d) A second insulating film is formed to cover the capacitor.
(e) A contact hole for contacting one of the lower and upper electrodes of the capacitor is formed by selectively removing the second insulating film by a dry etching process using an etching gas containing no hydrogen nor plasma.
The etching gas has a property that no hydrogen is generated through decomposition of the etching gas during the dry etching process.
With the method of fabricating a semiconductor device according to the second aspect of the present invention, the contact hole for contacting one of the lower and upper electrodes of the capacitor is formed by selectively removing the second insulating film by a dry etching process usingian etching gas containing no hydrogen. The etching gas has a property that no hydrogen is generated through decomposition of the etching gas during the dry etching process.
As a result, the dielectric or ferroelectric film of the capacitor is not reduced by the activated hydrogen existing in the atmosphere during the dry etching process. Thus, the leakage current is prevented from increasing and the dielectric breakdown resistance is prevented from degreasing during the dry etching process for forming the contact hole. This means that the performance degradation of the capacitor is prevented from occurring.
Moreover, when the capacitor has a ferroelectric film, in other words, the capacitor is a ferroelectric capacitor, the dielectric or residual polarization of the ferroelectric capacitor is prevented from degrading. This is also because the dielectric or ferroelectric film is not reduced by the activated hydrogen existing in the atmosphere during the thermal CVD process.
In a preferred embodiment of the method according to the second aspect of the present invention, the second insulating film is SiO2, and the etching gas is comprised of a composition of carbon (C) and fluorine (F). For example, CF4 and C2F6 may be used. Oxygen may be added to CF4 or C2F6.
In another preferred embodiment of the method according to the second aspect of the present invention, the second insulating film is SiNx, the etching gas is comprised of a composition of carbon (C) and fluorine (F). For example, CF4 and SiF4, or (NF3+Cl2) may be used. Oxygen may be added to CF4. Oxygen and nitrogen may be added to CF4.
As the dielectric film having a higher dielectric constant than SiO2 and Si3N4, an oxide of a single metal such as Ta2O5 may be used.
As the ferroelectric film, any ferroelectric film such as FZT, PLZT, SBT (SrBi2Ta2O9), and BTO(BaTiO3) may be used.